With transistor sizes becoming smaller and smaller and switching speeds faster and faster, transmission delay through interconnections between active devices is a major cause of concern. The delay depends on the resistance-capacitance and is referred to as RC delay. The resistance part of the signal delay is due to the resistivity of the metallic interconnect while the capacitance depends on the dielectric constant of the medium applied between the conductor lines. In order to reduce the dielectric constant SiO2 is being replaced by various low-k materials. In an effort to further reduce the dielectric constant these new materials are used in a porous form and since the dielectric constant of air is 1, ultimately some designs have part of the dielectric replaced by air (air bridges). This brings with it the increased need to protect the metal conductor from the ambient.
A few years ago Al—Cu alloy interconnects were replaced by copper interconnects due to the lower restitivity of copper, that is 1.68 micro Ohm-cm versus the 2.65 micro Ohm-cm resistivity of unalloyed aluminum. Copper has however a very great propensity for oxidation and the Cu2O that forms up to 300° is not a barrier to further oxidation having an open crystal structure.
Consequently copper is protected by barrier layers and capping layers. As the line width of interconnects decreases, the thickness of the layers protecting the copper lines in all directions becomes a significant fraction of the copper line width. Since these protective layers are neither good conductors nor low-k dielectrics, some of the gain in using a lower resistivity conductor and low-k dielectrics is lost.
Also, the National Technology Roadmap for Semiconductors predicts that for the 100 nm device dimension node, a material with a dielectric constant of about 2.0 will be needed to reduce the RC delay associated with BEOL interconnects. The most promising approach to developing such materials is to introduce porosity into a dense dielectric material. One of the problems of integrating these dielectrics into copper or silver based dual damascene schemes is that exposure to ambient conditions even at a relatively low temperature tends to cause oxide and sulphide formation impairing the conductivity of these originally high conductivity metals.